(a) Field of the Invention
The present invention relates to a method for preparing a glass substrate for use in making a magnetic recording medium and more particularly to a method for preparing a glass substrate for use in making a magnetic recording medium, which has a uniform surface, does not substantially have any surface defect and has a smooth surface having an extremely low surface roughness.
(b) Description of the Prior Art
There has been tried to reduce the flying height in order to improve the recording density of recording media in response to a substantial increase in the storage capacity of the magnetic disk recording unit. Thus, there has been required for the development of a substrate for use in making a magnetic recording medium, which is excellent in the surface smoothness and has a substantially small number of surface defects, in order to reduce the flying height.
Conventionally, a substrate prepared by plating an aluminum alloy with Nixe2x80x94P and then subjecting the principal face thereof to a multi-stage polishing operation has mainly been employed as a substrate for use in making a magnetic recording medium, which is excellent in the surface smoothness.
However, a magnetic disk recording unit has recently been adopted even in a portable personal computer such as a notebook-sized personal computer and this accordingly requires the development of a substrate for magnetic recording media capable of withstanding any impact encountered during carrying such a portable personal computer. In addition, the substrate for magnetic recording media should be rotated at a high speed of not less than 10000 rpm to increase the response speed of the magnetic disk recording unit and this correspondingly requires the use of a substrate for magnetic recording media having high strength. Thus, a glass substrate has been used as such a substrate satisfying the foregoing various requirements.
As such a glass substrate for magnetic recording media, there have principally been used a tempered glass substrate whose strength is enhanced by surface chemical strengthening and a crystallized glass substrate obtained by maintaining a glass substrate, which is prepared by melting and molding a raw glass material, at a high temperature ranging from 600 to 800xc2x0 C. over a long period of time to thus partially separate out crystalline phases therein.
The surface chemical strengthened glass substrate is a glass substrate obtained by, for instance, grinding and polishing a glass substrate capable of being chemically surface-strengthened (hereunder referred to as xe2x80x9cchemically surface-strengthenable glass substratexe2x80x9d) and obtained by melting and molding a raw glass material and then immersing the chemically surface-strengthenable glass substrate in a molten salt of, for instance, sodium nitrate or potassium nitrate to thus form a compressive stress-containing layer on the surface thereof and to in turn improve the breaking strength thereof. Moreover, the crystallized glass substrate may be a glass substrate obtained by, for instance, allowing amorphous glass prepared by melting and molding a raw glass material to stand at a high temperature ranging from 600 to 800xc2x0 C. over a long period of time to give a mixed phase in which 40 to 80% of a crystalline phase and 20 to 60% of an amorphous phase coexist.
If such a crystallized glass substrate or a chemically surface-strengthenable glass substrate is polished according to the well-known technique or using a soft abrasive cloth such as suede-type one along with an abrasive liquid containing a CeO2-base abrasive having an average particle size of about 0.5 to 2 xcexcm in an amount of about several % by mass, in the process for grinding and polishing, in particular, final polishing the glass substrate, however, brush mark-like linear unevenness is generated due to the use of the abrasive cloth and therefore, only glass substrates having uneven surface are obtained. Moreover, the lowest possible surface roughness accomplished by this technique is on the order of 3.5 xc3x85 for the chemically surface-strengthenable glass substrate and about 4.5 xc3x85 for the crystallized glass substrate. Consequently, it has been quite difficult to achieve a surface roughness of not more than 3 xc3x85, which is required for the glass substrate for use in making magnetic recording media.
It is thus an object of the present invention to provide a method for efficiently preparing a glass substrate for use in making a magnetic recording medium whose polished surface is quite uniform, which is substantially free of any surface defect and has a smooth surface having an extremely low surface roughness.
The inventors of this invention have conducted various studies to achieve the foregoing object, have unexpectedly found that a glass substrate for use in making a magnetic recording medium, which has a uniform surface free of any brush mark-like linear unevenness, which is substantially free of any surface defect and which has a surface roughness of not more than 3 xc3x85 can easily be prepared by polishing a raw glass substrate using an abrasive cloth, preferably a hard abrasive cloth of a foamed polyurethane in combination with an abrasive liquid which contains an ultrafine CeO2-base abrasive in a quite low concentration in a final polishing step, after lapping the glass substrate to a desired thickness according to the well-known method and, if necessary, polishing the substrate and have thus completed the present invention.
According to the present invention, there is thus provided a method for preparing a glass substrate for use in making a magnetic recording medium, which is characterized in that an abrasive liquid containing a CeO2-base abrasive having an average particle size, D50, of not more than 0.5 xcexcm in an amount of not more than 1% by mass and an abrasive cloth are used in a step for final polishing the glass substrate.
The present invention will hereunder be described in more detail.
The glass substrates, to be processed, used in the method for preparing a glass substrate for use in making a magnetic recording medium are not restricted to specific ones insofar as they have sufficient strength required for the glass substrate for magnetic recording media or they can acquire sufficient strength required for the glass substrate through any surface chemical strengthening treatment. However, it is preferred to use crystallized glass substrates and chemically surface-strengthenable glass substrate while taking into consideration the present situation of the techniques.
The abrasive used in the final polishing step of the method according to the present invention comprises CeO2-base fine particles whose average particle size: D50 is not more than 0.5 xcexcm, preferably not more than 0.3 xcexcm and more preferably not more than 0.2 xcexcm. This is because if the glass substrate is polished using an abrasive liquid containing an abrasive having an average particle size of higher than 0.5 xcexcm, the resulting glass substrate has an increased surface roughness. Therefore, the use of such an abrasive liquid cannot provide any desired glass substrate, which is substantially free of surface defects and has a smooth surface having an extremely low surface roughness.
As the CeO2-base abrasive usable in the final polishing step of the method of the present invention, there may be listed, for instance, high purity CeO2, mixtures of CeO2 with oxides of rare earth elements, mixture of CeO2 and SiO2, solid solutions consisting of cerium oxide and silicon oxide such as those disclosed in U.S. Pat. No. 5,766,279. Such a solid solution can be commercially available in the form of desired ultrafine particles and can particularly suitably be used in the method of the present invention because of their excellent dispersibility in water. Such a solid solution preferably consists of 100 parts by mass of cerium oxide and 0.1 to 10 parts by mass of silicon oxide.
The abrasive liquid used in the final polishing step in the production method according to the present invention comprises not more than 1% by mass and preferably not more than 0.5% by mass of the foregoing CeO2-base abrasive. If the glass substrate is polished using an abrasive liquid containing such a CeO2-base abrasive in an amount of higher than 1% by mass, the resulting glass substrate has an increased surface roughness. Therefore, the use of such an abrasive liquid cannot provide any desired glass substrate, which is substantially free of surface defects and has a smooth surface having an extremely low surface roughness. The reason why the surface roughness of the glass substrate is extremely low when the substrate is polished using an abrasive liquid whose CeO2-base abrasive concentration is very low has not yet clearly been elucidated at this stage, but it would be believed that the concentration of an abrasive required and effective for the polishing is in fact very low and that the presence of an excess of an abrasive in the abrasive liquid may adversely affect the surface roughness.
In the method of the present invention, the resistance to polishing and resistance to processing are high because of the low concentration of an abrasive in the abrasive liquid. However, these resistances can be reduced by the incorporation of a polymeric additive and/or a surfactant into the abrasive liquid like the conventional technique. Examples of such polymeric additive usable herein are celluloses and glycerin and examples of such surfactants usable herein are anionic, cationic and nonionic surfactants, with fatty acid type surfactants (soap) being preferably used. The use of these additives not only permits the reduction of these resistances, but also makes, more easier, the production of a glass substrate having a smooth surface and an extremely low surface roughness.
In the final polishing step of the method according to the present invention, a glass substrate is polished using a conventional double-sided polishing machine and a known abrasive cloth. The abrasive cloth is not particularly restricted to any specific one, but it is preferred to use a rather hard abrasive cloth in order to enhance the mechanical action among the mechano-chemical actions during polishing. As such rather hard abrasive cloths, preferred are hard abrasive cloths made of foamed polyurethanes which are conventionally used in a coarse polishing (primary polishing) step. The hardness of the hard abrasive cloth of a foamed polyurethane is classified into several stages and those classified into any stage may successfully be used in the present invention. However, preferably used are those having a higher hardness from the viewpoint of preventing any corner slope of the glass substrate. Moreover, it is preferred to form grooves having a width of 1 mm and a depth of 1 mm at intervals of 30 to 50 mm on the polishing side of the hard abrasive cloth to make the flow of the abrasive liquid easy. Furthermore, the thickness of the abrasive cloth preferably ranges from about 0.5 to 1.5 mm.
As has been discussed above, if a glass substrate is polished using a suede type soft abrasive cloth and an abrasive liquid containing a CeO2-base abrasive having an average particle size of about 0.5 to 2 xcexcm in an amount of about several percentages, brush mark-like linear unevenness having directional qualities is formed because of the polishing with an abrasive cloth and such a polishing operation may only provide a glass substrate having a non-uniform surface. On the other hand, the occurrence of such brush mark-like linear unevenness due to the use of such an abrasive cloth can be suppressed by the use of an abrasive liquid containing not more than 1% by mass of a CeO2-base abrasive having an average particle size, D50, of not more than 0.5 xcexcm. Moreover, any occurrence of such brush mark-like linear unevenness due to the use of such an abrasive cloth can be further inhibited by the simultaneous use of such an abrasive liquid and a hard abrasive cloth of a foamed polyurethane as specified above. Such brush mark-like linear unevenness due to the use of such an abrasive cloth comprises extremely small projections which cannot be recognized by the usual visual inspection of the appearance of the glass substrate, but they can visually recognized if a magnetic film is deposited thereon. In this respect, the brush mark-like linear unevenness due to the use of the abrasive cloth described above can be reduced by the use of a combination of a hard abrasive cloth and an abrasive liquid containing the usual CeO2-base abrasive having an average particle size ranging from about 0.5 to 2 xcexcm, but fine surface defects are formed in this case.
In the method of the present invention, the conditions for polishing other than those discussed above such as polishing pressure, number of rotations and polishing time are not restricted to specific ones insofar as the usual polishing step can be performed, without any trouble. However, if the polishing pressure is too high, the abrasive cloth makes a sound of creaking and/or the glass substrate is damaged. Therefore, the polishing step is carried out at a polishing pressure of preferably less than 100 g/cm2, and more preferably not more than 80 g/cm2. Regarding the number of rotations, if it is too low, the polishing machine makes a noise of creaking, while it is too high, the polishing resistance and the processing resistance are extremely increased. Thus, the number of rotations preferably limited to the range of from about 15 to 60 rpm. In addition, the polishing time is closely related to the number of rotations, but if it is too short, the improvement of the surface roughness is liable to be insufficient, while if it is too long, any further improvement of the desired effect in proportion thereto is not expected any more. For this reason, the polishing time is preferably adjusted to the range of from about 150 to 1000 as expressed in terms of the product: polishing time (min)xc3x97the number of rotations (rpm).
Thus, a crystallized glass substrate for use in making a magnetic recording medium, which has an extremely low surface roughness and a smooth surface can easily be prepared by polishing a raw crystallized glass substrate or a chemically surface-strengthenable glass substrate, according to the method of the present invention.